Earth
'''Earth''' '''Earth''' is the third [http://en.wikipedia.org/wiki/Planet planet] from the [http://en.wikipedia.org/wiki/Sun Sun], and the [http://en.wikipedia.org/wiki/Density densest] and fifth-largest of the eight planets in the [http://en.wikipedia.org/wiki/Solar_System Solar System]. It is also the largest of the Solar System's four [http://en.wikipedia.org/wiki/Terrestrial_planet terrestrial planets]. It is sometimes referred to as the [http://en.wikipedia.org/wiki/World world], the Blue Planet,[http://en.wikipedia.org/wiki/Earth#cite_note-blueplanet-27 [22]] or by its Latin name, ''[http://en.wiktionary.org/wiki/Terra Terra]''.[http://en.wikipedia.org/wiki/Earth#cite_note-Terra-28 [note 6]] Earth formed [http://en.wikipedia.org/wiki/Age_of_the_Earth approximately 4.54 billion years] ago, and [http://en.wikipedia.org/wiki/Abiogenesis life appeared] on its surface within one billion years.[http://en.wikipedia.org/wiki/Earth#cite_note-age_earth1-29 [23]] Earth's [http://en.wikipedia.org/wiki/Biosphere biosphere] then significantly altered [http://en.wikipedia.org/wiki/Atmosphere_of_Earth the atmospheric] and other [http://en.wikipedia.org/wiki/Abiotic_component basic physical] conditions, which enabled the proliferation of [http://en.wikipedia.org/wiki/Aerobic_organism organisms] as well as the formation of the [http://en.wikipedia.org/wiki/Ozone_layer ozone layer], which together with [http://en.wikipedia.org/wiki/Earth%27s_magnetic_field Earth's magnetic field] blocked harmful [http://en.wikipedia.org/wiki/Sunlight solar radiation], and permitted formerly ocean-confined life to move safely to land.[http://en.wikipedia.org/wiki/Earth#cite_note-Harrison_2002-30 [24]] The [http://en.wikipedia.org/wiki/Geophysics physical properties of the Earth], as well as its [http://en.wikipedia.org/wiki/Geology geological] history and orbit, have allowed life to persist. Estimates on how much longer the planet will be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).[http://en.wikipedia.org/wiki/Earth#cite_note-britt2000-31 [25] ][http://en.wikipedia.org/wiki/Earth#cite_note-carrington-32 [26]][http://en.wikipedia.org/wiki/Earth#cite_note-pnas1_24_9576-33 [27]] Earth's [http://en.wikipedia.org/wiki/Lithosphere lithosphere] is divided into several rigid segments, or [http://en.wikipedia.org/wiki/Plate_tectonics tectonic plates], that migrate across the surface over periods of [http://en.wikipedia.org/wiki/Geologic_time_scale many millions of years]. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the [http://en.wikipedia.org/wiki/Hydrosphere hydrosphere]. Earth's [http://en.wikipedia.org/wiki/Geographical_pole poles] are mostly covered with ice that is the solid ice of the [http://en.wikipedia.org/wiki/Antarctic_ice_sheet Antarctic ice sheet] and the [http://en.wikipedia.org/wiki/Sea_ice sea ice]that is the [http://en.wikipedia.org/wiki/Polar_ice_packs polar ice packs]. [http://en.wikipedia.org/wiki/Structure_of_the_Earth The planet's interior] remains active, with a solid iron [http://en.wikipedia.org/wiki/Inner_core inner core], a liquid [http://en.wikipedia.org/wiki/Outer_core outer core] that generates the magnetic field, and a thick layer of relatively solid [http://en.wikipedia.org/wiki/Mantle_(geology) mantle]. Earth [http://en.wikipedia.org/wiki/Gravitational_interaction gravitationally interacts] with other objects in space, especially the Sun and the [http://en.wikipedia.org/wiki/Moon Moon]. During one orbit around the sun, the Earth rotates about its own axis 366.26 times, creating 365.26 [http://en.wikipedia.org/wiki/Solar_time solar days], or one [http://en.wikipedia.org/wiki/Sidereal_year sidereal year].[http://en.wikipedia.org/wiki/Earth#cite_note-sidereal_solar-34 [note 7]] The Earth's axis of rotation is [http://en.wikipedia.org/wiki/Axial_tilt tilted] 23.4° away from the [http://en.wikipedia.org/wiki/Perpendicular perpendicular] of its [http://en.wikipedia.org/wiki/Orbital_plane_(astronomy) orbital plane], producing seasonal variations on the planet's surface with a period of one [http://en.wikipedia.org/wiki/Tropical_year tropical year] (365.24 solar days).[http://en.wikipedia.org/wiki/Earth#cite_note-yoder1995-35 [28]] The Moon is Earth's only [http://en.wikipedia.org/wiki/Natural_satellite natural satellite]. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean [http://en.wikipedia.org/wiki/Tide tides], stabilizes the axial tilt, and gradually slows the planet's rotation. The planet is home to millions of [http://en.wikipedia.org/wiki/Species species], including [http://en.wikipedia.org/wiki/Human humans].[http://en.wikipedia.org/wiki/Earth#cite_note-science_241_4872_1441-36 [29]] Both the [http://en.wikipedia.org/wiki/Mineral mineral] resources of the planet and the products of the [http://en.wikipedia.org/wiki/Biosphere biosphere] contribute resources that are used to support a [http://en.wikipedia.org/wiki/World_population global human population].[http://en.wikipedia.org/wiki/Earth#cite_note-World_Population_Clock-37 [30]] These inhabitants are grouped into about 200 independent [http://en.wikipedia.org/wiki/Sovereign_state sovereign states], which interact through diplomacy, travel, trade, and military action. [http://en.wikipedia.org/wiki/Culture Human cultures] have developed many views of the planet, including its [http://en.wikipedia.org/wiki/Anthropomorphism personification] as a planetary[http://en.wikipedia.org/wiki/Deity deity], its shape [http://en.wikipedia.org/wiki/Flat_Earth as flat], its position as [http://en.wikipedia.org/wiki/Geocentric_model the center of the universe], and in the modern [http://en.wikipedia.org/wiki/Gaia_hypothesis Gaia Principle], as a single, self-regulating organism in its own right.